The growth rate and body composition of lambs depend on their rate of feed intake and the composition of the feed given. The purpose of the experiment described here was to test predictions of the growth and composition of lambs given feeds of two protein contents at three levels of feeding.

73 Scottish Blackface wethers were abruptly weaned at 49 days of age and an average live weight (LW) of 18 (sd 2.2) kg. Six, chosen to cover the range in initial liveweight, were killed and those remaining were put into individual pens and fed in one of six ways. Two feeds with 140g CP/kg fresh weight (FW), called LP, and 220 g CP/kg FW, called HP, were used. They were formulated using barley, maize and soyabean meal, and hay at 150g/kg FW, and were calculated to yield 11.8 MJ ME/kg FW. Each feed was given ad libitum (AL) and at daily rates of 1.2 or 0.8 kg FW. The numbers of sheepstarted were: ALHP, n = 8; ALLP, n = 8; 1.2 HP, n = 12, 1.2 LP, n = 13, 0.8 HP, n = 14, 0.8 LP, n = 12. Sheep were killed from each treatment at 5 kg LW intervals from 25 to 50 kg. After slaughter the weights of gut-fill and organs were taken and the carcase physically dissected into lean, bone and fat. The chemical composition - dry matter and the ash, N, lipid and gross energy contents of the dry matter - of the whole empty body, excluding wool, was measured. Wool weights were determined. The intakes and weights of each sheep were measured twice weekly.